Although aqueous emulsion paints have been known and used for decades, recent research and development efforts have been directed to water-based compositions which perform comparable to solvent-based paints in weathering, durability, and flexibility. Such activity arises out of concerns for the pollution and health hazards associated with the use of solvent-based paints. The United States Environmental Protection Agency has set 1991 as the date for restricting solvent-based paints for commercial use.
Paints as coatings on substrates have long been used for appearance and for protection against weathering as well as, for example, safety insulation, and vapor barrier. Paints are generally considered to be composed of four basic components: pigment(s), binder (or nonvolatile vehicle), volatile vehicle (or carrier), and additives. Pigments, which may be either organic or inorganic compositions, supply the desired color of a paint and are selected for proper opacity and gloss. The binder is a substance which, when exposed to the atmosphere or heat, forms a dry coating or film, and provides the medium for the pigment. Binders are typically resins (often synthetic polymeric materials), drying oils, or mixtures of such materials. The volatile vehicle may make up to 50% of the volume of the paint, but is vaporized into the atmosphere when the paint is dried or cured. The volatile vehicle in solvent-based (also called solvent-borne) paints is typically an organic solvent, such an aromatic hydrocarbon (e.g., xylene or toluene) or an aliphatic hydrocarbon (e.g., mineral spirits or naphtha), while the volatile vehicle in water-based (also called water-borne) paints is, of course, water. In the largest group of water-based paints, the binder is emulsified into the water medium, i.e., the binder is dispersed as tiny droplets in the water, the binder being the internal phase and the water being the external phase. These paints are also referred to as emulsion paints or emulsion-based paints.
Additives are agents used to facilitate acceptable film formation. Additives for a typical water-based paint include coalescents, thickeners, defoamers, preservatives, pH controllers, and anti-freezes. Coalescents are typically added to plasticize the binder temporarily during film formation so that the emulsion particles coalesce. Thickeners are often added to promote suspension of the pigment during storage, proper rheology for application, and flow without sagging. Preservatives are often added for protection during storage against bacterial attack, while the other additives are added to minimize foaming, adjust pH, prevent surface defects, and provide freeze/thaw stability.
The hobby, model and toy industry has been an area where very little choice is available for water-based paints. Since the 1930's, the hobby industry has generally had to accept the coatings tecnology used in other industries, for example, automotive lacquers and enamels. When lead pigments were required to be replaced with more user and environmentally friendly alternatives, many of the hobby paints were removed from the market. Only a limited number of companies continued to bottle solvent-borne hobby coatings.
Only after the federal Environmental Protection Agency (and California Air Quality Control Council) took steps to alert the general public to the dangers involved in the constant use of solvent-borne hobby paints, did the industry attempt to respond with water clean up or water-borne paints. These formulations were at best inadequate, providing poor opacity and adhesion characteristics. While water-borne paints have continued to find acceptance in the art materials area, providing acceptable adhesion for canvas and paper substrates, the hobbyist/modeler found that he or she was unable to successfully use these coatings over the wide variety of substrates commonly encountered. Solvent based paints still remain the standard in the art.
Meanwhile, the hobby industry continues to advance in areas of manufacturing technology and the ability to replicate the very fine detail from the originally modeled object. Contemporary models have very accurate details that are of a very small scale, often down to 1/250 scale. Suitable coatings and paints must provide thin films which do not hide the details of the model, yet provide good adhesion, flexibility and impact resistance. Despite recognition of superior safety and environmental properties of water-borne coatings, the art has yet to respond with a water-based coating which can provide the very thin paint film needed to accent the extreme detail of metal and plastic models.